Depending on the context, you could use:
* Crib: If you're talking about a place for a baby to sleep, the opposite of a coffin would be a crib.
* Life: This is a broader, more conceptual opposite, referring to the state of being alive as opposed to dead.
* Birth: This is also a broader concept, referring to the beginning of life as opposed to the end.
* Memorial: If you're focusing on the act of remembering someone, a memorial could be seen as the opposite of a coffin, which represents the finality of death.
Other possibilities:
* Casket: While this is technically a synonym, it's less formal and could be considered an antonym in some contexts.
* Urn: This is a container for ashes, so it could be seen as an opposite of a coffin in the sense of being a different way to contain a deceased person.
Ultimately, the best antonym will depend on the specific situation and what you want to convey.